Recently, in line with miniaturization, lightweight, thin profile, and portable trends in electronic devices according to the development of information and telecommunications industry, the need for high energy density batteries used as power sources of such electronic devices has increased. Currently, research into lithium secondary batteries, as batteries that may best satisfy the above need, has actively conducted.
Graphite is mainly used as an anode material of the lithium secondary battery. However, graphite has a low capacity per unit mass of 372 mAh/g and a high-capacity lithium secondary battery may be difficult to be prepared by using graphite.
However, since a silicon-based material has a capacity (4,190 mAh/g) 11 times or more higher than a theoretical capacity (372 mAh/g) of a carbon-based anode active material, the silicon-based material is on the spotlight as a material for replacing the carbon-based anode active material. However, since volume expansion of the silicon-based material during the intercalation of lithium ions is 3 times or more when silicon is only used, the capacity of a battery tends to decrease as charging and discharging of the battery proceed and safety issues may also occur. Thus, many techniques are required to commercialize the silicon-based material.
Therefore, a significant amount of research into an increase in the capacity of an anode active material, such as silicon, i.e., a decrease in a volume expansion coefficient by alloying of silicon, has been conducted. However, since a metal, such as silicon (Si), tin (Sn), and aluminum (Al), is alloyed with lithium during charge and discharge, volume expansion and contraction may occur. Thus, cycle characteristics of the battery may degrade.
Although silicon is known as an element that may most likely provide high capacity, it may be very difficult to amorphize silicon itself alone and it may be also difficult to amorphize an alloy including silicon as a main component. However, a method of easily amorphizing a silicon-based material has recently been developed by using mechanical alloying.
For example, as a method of preparing an anode active material for a lithium secondary battery using a silicon alloy, a method of preparing an anode active material has been developed, in which powders of a silicon element and an element M (where M is nickel (Ni), cobalt (Co), boron (B), chromium (Cr), copper (Cu), iron (Fe), manganese (Mn), titanium (Ti), or yttrium (Y)) are alloyed by mechanical alloying to form a SiM alloy, the SiM alloy is heat treated, and the heat-treated SiM alloy is then alloyed with powder of an element X (where X is silver (Ag), copper (Cu), and gold (Au)) by mechanical alloying to obtain a SiMX alloy.
However, with respect to the anode active material for a lithium secondary battery prepared by the above method, its charge and discharge capacity may be decreased due to the degradation of silicon as charge and discharge cycles proceed. With respect to the mechanical alloying, since the destruction of an alloy structure may occur due to the intercalation and deintercalation of lithium, the cycle characteristics may degrade.
Therefore, there is a need to develop an anode active material which may replace a typical anode active material and may improve discharge capacity, efficiency, and life characteristics when used in the lithium secondary battery.